As a conventional process of producing N-substituted carbazoles, there has been a process in which a carbazole compound is reacted with an alkyl halide in the presence of a quaternary ammonium salt such as benzyltriethylammonium chloride using 50% aqueous sodium hydroxide as an acid-removing agent (Bull. Chem. Soc. Jpn., 54, 1879 (1981)). This process, however, requires the use of a certain quaternary ammonium salt which is relatively expensive, and it can attain only a low yield, even if the alkyl halide used is relatively reactive, such as ethyl bromide, methyl iodide or allyl chloride; therefore, it cannot always be said that this process is suitable for the production on an industrial scale.